1. Field of the Invention
The present invention relates to a layered low-pass filter including two coils that are provided within a layered substrate and connected in series to each other.
2. Description of the Related Art
Recently, there have been strong demands for reductions in size and thickness of electronic apparatuses for portable use typified by cellular phones and notebook personal computers, and accordingly, reductions in size and thickness have also been required for electronic components for use in such electronic apparatuses. Low-pass filters are one of the electronic components for use in such electronic apparatuses. Reductions in size and thickness are also required for low-pass filters. To meet this, it has been proposed to configure a low-pass filter using a layered substrate, as disclosed in JP 2002-204136A and JP 2003-158437A, for example. A low-pass filter configured using a layered substrate as mentioned above will be hereinafter called a layered low-pass filter.
To obtain a steep attenuation characteristic with a low-pass filter, the low-pass filter should preferably be of the fifth order or higher. A fifth-order low-pass filter includes, for example, a first coil and a second coil connected in series to each other, a first capacitor provided between the ground and the node between the two coils, a second capacitor provided between the ground and an end of the first coil opposite to the node, and a third capacitor provided between the ground and an end of the second coil opposite to the node.
Elliptic (simultaneous Chebyshev) low-pass filters are known as being capable of providing a steeper attenuation characteristic. Elliptic low-pass filters present at least one attenuation pole in the stop band in the pass attenuation characteristic. JP 2002-204136A discloses a layered low-pass filter having a circuit configuration of a fifth-order elliptic low-pass filter. The layered low-pass filter disclosed in JP 2002-204136A presents two attenuation poles in the stop band in the pass attenuation characteristic.
JP 2003-158437A discloses a layered LC filter including a low-pass filter circuit and a trap circuit. In this LC filter, the low-pass filter circuit has two inductors connected in series between an input terminal and an output terminal. Each of the inductors is configured by connecting a columnar inductor formed using via holes and a coil conductor pattern in series to each other. The trap circuit has an inductor and a capacitor connected in series between the ground and the node between the two inductors of the low-pass filter circuit. The inductor of the trap circuit is formed using via holes.
For devices incorporating low-pass filters, there may be cases where it is required that the spurious, such as harmonics, of signals passing through the low-pass filter be reduced. In such cases, a notch filter for reducing the spurious can be connected in series to the low-pass filter. However, this causes the problem of an increase in insertion loss in the pass band of the low-pass filter.
A possible solution to this is to employ a type of low-pass filters that present attenuation poles like the elliptic type and make use of the attenuation poles to thereby reduce the spurious. When an attempt is made to implement this with a layered low-pass filter having a plurality of coils connected in series, the following problem is encountered, however. In a layered low-pass filter having a plurality of coils connected in series, as disclosed in JP 2002-204136A, coupling occurs between the coils. The coupling occurring between the coils is magnetic field coupling. If magnetic field coupling occurs between the coils, the two attenuation poles become farther from each other, compared with a case where no magnetic field coupling occurs between the coils. This makes it difficult to increase attenuation at frequencies at which a greater attenuation is desired.
In the layered LC filter disclosed in JP 2003-158437A, magnetic field coupling presumably occurs between the two coil conductor patterns. On the other hand, there seems to occur no magnetic field coupling between the two columnar inductors in this LC filter because a capacitor pattern connected to the ground is provided between the two columnar inductors.